Lung surfactant lipoprotein has been prepared from lavages of rabbit and dog lungs. Disc gel electrophoresis has shown that lung surfactant contains two apoproteins with masses of 10,000 daltons and 30,000 daltons, respectively. Spin label studies indicated that surfactant is highly ordered at the aqueous interface but that the interior of the lipoprotein is highly fluid in the interior of the lipoprotein. Nuclear magnetic resonance measurements have demonstrated that the motion of the phosphate groups in surfactant is highly hindered.